The present invention relates to a method and an arrangement for refreshing a frame memory in an interframe encoding system.
In a television system, thirty frames of video signals are transmitted in one second, with a 33 msec. time interval between two successive frames. There is little variation from one frame to the other. In the particular case of video signals televising a conference (hereinafter referred to as "TV conference signals"), video signals of two successive frames differ very little since participants very rarely exhibit motions. With such video signals, considerable compression of frequency bandwidth is effected by taking a difference between two frames and transmitting only the difference from the areas of motions. This is known as an "interframe encoding system."
In the prior art interframe encoding system, a frame of video signals is segmented into blocks, each of which has a size of m lines x n picture cells (pixels). When all the pixels in one block are, for example, static, the block is then considered as being ineffective and an ineffective block signal is transmitted. On the other hand, when a block contains areas of motion, the block is considered as being effective and an effective block signal is transmitted. At the same time, the difference between the pixels contained in the effective block is quantized and encoded to be transmitted to the receiving side. Details of such a block encoding system are available in the conference record of the International Conference on Digital Satellite Communications, 1975, pp. 309-314, under the title of "H-1 NETEC SYSTEM: INTERFRAME ENCODER FOR NTSC TELEVISION SIGNAL" (reference 1) by T. Ishiguro et al.
In the prior art interframe encoding system, as described above, the interframe difference alone is transmitted. At the receiving side, the interframe difference is added to the content from a frame memory to reproduce the original video signal. If a code error occurs in a transmission line, the original video signal cannot be reproduced precisely at the receiving side because the interframe difference which is transmitted differs from the interframe difference received at the receiving side. The effect of such a code error in the transmission line continues until the contents of the frame memories at the transmitting and receiving sides become identical. It is, therefore, necessary to refresh the contents of the frame memories with updated data. Such a technique is generally termed as the refreshing of a frame memory.
The conventional refreshing methods can roughly be classified into two classes:
(i) A "demand" refreshing method is one in which the contents of the frame memories of both transmitting and receiving sides are refreshed (1) by transmitting, via an additional transmission line, a refreshing demand signal, and (2) by synchronizing the refreshing of the two frame memories when a code error on the transmission line is detected at the receiving side.
(ii) A "periodic" refreshing method in which a command signal, instructing a refreshing of the frame memory at the receiving side, is supplied periodically from the transmitting side at given intervals irrespective of code error occurrences on the transmission line.
However, since the demand refreshing method requires an additional transmission line for sending the refreshing demand signal from the receiving side to the transmitting side, the demand method cannot be employed in a one-way communication system which is not provided with such an additional line. A periodic refreshing method is described in U.S. Pat. No. 4,051,530 (reference 2). The difference between the pixels is taken for each pixel, thereby requiring an enormous amount of refreshing data to finish the refreshing of all the frame memory contents. It has, therefore, been a common practice to limit the number of pixels per frame to be refreshed to, for example, those pixels required for five lines. However, this common practice is still defective since a large amount of the refreshing data for pixels in five lines will be produced at one time. Thus, a necessary video signal which is indicative of the movements of an object cannot be adequately transmitted. For example, assume that the refreshing data consists of an 8 bit PCM data per pixel, the number of pixels per line is 455, and the number of lines to be refreshed per frame is five lines. Then, the refreshing data amounting to as much as 5.times.455.times.8=18,200 bits will occur all at once, in a burst. That amount corresponds to approximately 36% of 50 Kbits, which is the approximate transmission capacity per frame period, when a video signal is transmitted at 1.5 Mb (thirty frames)/s, making it difficult to transmit sufficient motion of the object, as mentioned above.